1. Field Of The Invention
This invention relates generally to metal working and more particularly to processes of mechanical manufacture for electrical devices such as dynamo-electric machines.
2. Description Of The Prior Art
During the manufacture of electric motors, stator cores are preassembled. These cores generally comprise a stack of laminations which are usually individually punched from coil stock. The stock material is generally referred to as a magnetic type of electrical steel.
Such stator cores are annealed for the purpose of developing a larger grain size and thus optimizing the magnetic properties of the core material. Sometimes the individual laminations are separately annealed and other times a core is fabricated into a stack of laminations and then annealed as a unit.
Another purpose of the annealing process is for forming an iron oxide scale on the surface of each lamination resulting in desirable inter-lamination resistance properties, that is, eddy current losses between laminations are limited.
Physically, stator cores are of various outside configurations, for example, round, square, rectangular or similar shapes. A circular stator bore is formed in each lamination regardless of the outside configuration of the core. Thus, when the laminations are stacked and the bores aligned, a cylindrical bore is formed in the core. Also, coil slots are formed in each lamination in circular disposition about the circumference of the bore. A portion of each slot actually intersects the bore. Slot depth varies to give a desired flux density characteristic in the core. Between adjacent slots is a portion of the lamination which forms the inside diameter of the bore. These portions are called stator teeth and they transmit the desired flux from the stator core to the rotor which rotates within the bore when the motor is assembled. Since the rotor rotates in proximity to the stator bore, there being only a slight air gap between the outside diameter of the rotor and the inside diameter of the bore, uniformity in the bore circumference is critical.
When laminations are stacked to form the fabricated stator core, several factors contribute to the lack of uniformity in the circumference of the stator bore. Contributing to these factors are, for example:
1. NON-UNIFORM STRESSES IN THE COIL STEEL;
2. RELIEF OF THESE STRESSES WHEN THE LAMINATIONS ARE PUNCHED;
3. DISTORTION CREATED DURING THE ANNEALING PROCESS;
4. IRREGULARITIES DUE TO STAGGERED LAMINATIONS; AND
5. WELDING STRESSES DISTORTING THE CORE IF LAMINATIONS ARE WELDED TO FORM A UNIT.
To provide the critical uniformity to the bores or to size the bores as it is referred to, various techniques have been used. Some of these techniques are as follows:
1. HAND FILING THE INNER CIRCUMFERENCE OF THE BORE;
2. HAND GRINDING THE BORE;
3. MACHINING THE BORE WITH A CUTTING TOOL;
4. HONING;
5. BROACHING; AND
6. ROLLER BURNISHING.
Also, an oversized plug is sometimes forced directly into the bore resulting in direct contact between the plug and the bore as the plug forcibly shaves the inner circumference of the bore.
While these techniques do improve the uniformity of the bore circumference they create other undesirable problems in the core such as undesirable stresses created in the tips of the stator teeth which cause hysteresis losses in the stator material and such as shorting laminations at the bore circumference which destroys the desirable interlamination resistance created by the iron oxide scale. These problems can all be based on the fact that the above-described techniques used to improve bore circumference uniformity have a single common denominator. That denominator involves direct forcible contact between a moving member and the bore circumference.
It would be desirable to have a device or method capable of providing the desired uniformity of the bore circumference without involving the undesirable characteristics of presently known techniques.